Dogecoin is interesting. On one hand, it's all an elaborate joke and everyone involved knows it. On the other hand, it's got a higher transaction volume than its 'serious' cousin Bitcoin, and people occasionally use it to transfer significant amounts of money (e.g. here, and the Jamaican Bobsled Team).
The biggest problem a currency has to overcome is the network effect. It's damned hard to bootstrap a currency, to get people to start -- and continue -- to use it instead of some other currency. With conventional currencies, that's typically done by governments requiring taxes to be paid in it. This has the effect of basically guaranteeing that everyone also receives their income in it as well and makes it difficult for another currency to sneak in. Taxes basically anchor the national economy onto the national currency.
With crypto-currencies, that's not an option since there's no government to levy taxes. But Bitcoin did something similar through vices: the Silk Road and Satoshi Dice got people using Bitcoin and money into the economy because they wanted to use it for drug trafficking and gambling. Because both of those sites only accepted Bitcoin, and they're either illegal or highly restricted in the normal economy, they acted as an anchor to convince people to adopt Bitcoin and move money into and out of its economy. Once bootstrapped, it's now being used for other, more legitimate, purposes.
Dogecoin is bootstrapping itself in a different way, through indiscriminate tipping and charity. It's using this kind of frivolous spending as a way of getting its transaction volume up and bootstrapping exchanges between Dogecoin and Bitcoin and USD. I could see it moving up to micro-transactions of some kind -- something small, perhaps still a joke of some kind, but for transactions (i.e, receiving something in return) rather than donation. Who knows, maybe one day we all woke up, and without realizing it Dogecoin has gone from a joke, to a joke people use for real stuff around the edges, to a real currency with a joke around the edges?